GMC Repair: P1415 Code, and A/C Compressor wont turn on, r 134a, gmc sierra

Question
I have a 2000 GMC Sierra 1500 2WD 3door short bed, 5.3L, 4L60E Trans. its throwing the P1415 Code twice. I know its the Secondary Air Injection System (left bank). But why the same code twice and what could be wrong/how to fix, and could that be the reason why my oil preassure gauge needle vibrates at idle?  And my second question I bought a can of R-134a refriderant from auto zone and decided to charge my system due to it not blowing cold enough air. after doing so my compressor would kick on and then immediately off. cycling itself. upon releaving the system of some pressure to be sure it wasnt over pressurized and now it won't come on at all..all my fuses are good and so is the relay...What could possibly be the problem and what can I check out? Thank you soo much for your time..sorry for such a long email

Answer
Hi
I'll address all three issues.
1) p1415. As much as I'd like to tell you 1 single part is could be, the diagnostic worksheet requires a scan tool.
IF you have access to one, I'll be MOE than happy to help you with this. If not You'll be throwing parts at the truck and parts = $$$$$. If you want an opinion, I'd REALLY check the Secondary Air Injection Check Valve which is the diaphram located on the AIR pipe. (There is 2 of them, one for each bank, and located right on top in center of engine).
2) oil pressure = Depending on the flucation. If it is VERY rapid but basically stays around the ame point, I would say it is the cluster.
If it flucates heavily, say like from 80 psi to 40 or lower I would attach a manual gauge to it. If it still does flucation, you'll have a mechanical problem. If it does not flucate, I would GUESS the oil pressure sender unit.
3)A/C = Sounds like, to me, it is VERY low on freon.
You added 12 oz. The system holds 32oz (2#)12 oz is not enough. It will cycle VERY rapid and never get cold.
Here again, my best advice is unl;ess you really know what the pressures are, you are running the risk of damaging the compressor and or other A/c components. There are some fail safe built in, but they COULD be faulty too. Best advice, most major repair chains (meineke, midas, pepboys etc..) USUALLY offer a free A/C system check up. I would HIGHLY recommend taking it to one YOUR familiar with and letting them diagnose the system.
All this is just advice.
I Hope it helps.
Follow back post, if needed.
Pawl